Conventional electric double layer capacitors come in various types. Examples are coin and button types, in which the separator is interposed between the pair of electrodes, and this, together with the electrolyte, is sealed within a metal case, a sealing plate, and a gasket that insulates the two from each other; coil types, in which the electrodes and separator are coiled to produce an electric double layer capacitor unit, which is then placed in a metal case and impregnated with the electrolyte; and stacked types in which a plurality of rectangular electrodes and separators interposed between the electrodes are superposed in alternating fashion to produced an electrode laminate, a positive electrode lead is connected to the positive terminal and a negative electrode lead is connected to the negative terminal to produce a electric double layer capacitor unit, which is then placed in a metal case, impregnated with the electrolyte, and sealed.
Electric double layer capacitors for use in power applications such as automobiles must possess energy densities so as the achieve both high capacitance and low internal resistance, and must also exhibit high output densities. With the goal of meeting these requirements, research is being conducted into ways to produce thinner films and increase surface area so as to increase the opposing electrode area. It is also necessary for the electrodes to have integrated collectors. Mass production is a further requirement of electric double layer capacitor.
The electrodes used in conventional electric double layer capacitors are ordinarily manufactured by (1) a process in which a mixture of paste or ink form containing the electrode material is applied to the collector by coating or other means, dried (solvent removal), and then calendered or the like, or (2) a process in which a sheet which will serve as the electrode material is first prepared, and this is then integrated with the collector using a calender roll or the like. The largest drawback of these methods is the questionable ability to produce good contact between the collector and the electrode material powder. Techniques effective for producing an electric double layer capacitor having good contact between the collector and the electrode material and exhibiting low internal resistance include increasing the contact area and creating adequate interpenetration between the collector and the electrode material.
Specific examples of processes falling under (1) are a method in which a paste consisting of activated carbon powder, a fluoropolymer, and methyl alcohol is coated onto an aluminum net which serves as the collector (Japanese Laid-Open Patent Application 4-162510); a method in which a slurry prepared by adding an aqueous solution of carboxymethyl cellulose to a mixed solution containing activated carbon powder, acetylene black, water, and methanol is applied to the roughened surface of aluminum foil serving as the collector (Japanese Laid-Open Patent Application 4-162510); and a method in which a mixture of polyvinyl pyrrolidone and an aqueous dispersion of polytetrafluoroethylene is added as a binder to activated carbon powder and acetylene black, and this is applied to aluminum expanded metal which serves as the collector (U.S. Pat. No. 4,327,400). Specific examples of processes falling under (2) are a method in which an electrode material sheet is prepared from activated carbon powder, a conductive agent, and polytetrafluoroethylene used as binder to bind, the electrode material sheet is superposed on a collector consisting of a metal foil with a surface roughened by etching, an expanded metal, or other material, and is then fed through a calender roll to produce a thin electrode (Japanese Patent Publication 54-12620).
Since the electrode material is actually a powder aggregate, its surface may be imagined as having extremely tiny peaks and valleys. Accordingly, contact between the collector surface and the electrode surface occurs in a point-contact configuration. Where the surface of the metal foil serving as the collector has been roughened, the increased surface area reduces the internal resistance of the electric double layer capacitor.
However, contact area cannot be considered to reach adequate levels, even where the collector surface has been roughened. The bonding strength between the electrode material sheet and the collector is not adequate to withstand the feed tension and winding during continuous production of electrodes in roll form, and contact tends to weaken over time.
In a collector consisting of expanded metal or the like provided with openings, contact is improved by causing the electrode material to penetrate into the openings provided in the collector. However, increasing the opening size to facilitate penetration has the effect of reducing strength and reducing the unit cross section of the collector, thereby increasing resistance of the collector. Thus, the effect in reducing the internal resistance of the electric double layer capacitor is less than anticipated.
The present invention was developed with the foregoing in view, and is intended to provide an electric double layer capacitor exhibiting low internal resistance, provided with polarizable electrodes in which the collector and the electrode material are securely integrated in such as way as to produce adequate bonding strength and good contact.